Hi guys, i rly need Help i rly dont know how to help him, my friends told me that he need just keep feeding and change water every 7 days up to 20% of tank size. I never sawg something like that, u think its by bad water?

Hi.
Have you tested the water for ammonia, nitrite, nitrates and ph? You will need to get these measurements, but in the meantime, keeping the water clean, well filtered and highly oxygenated will help. Twice weekly water changes of 10-20% and deal with the results once you know them. Has anything changed in your tank? New fish? Change of food? What do you feed him and how long do you leave it in the tank? Looks like a fish you have had for along time?
Martin